Electrodeposition of thin-film cells containing non-toxic elements

ABSTRACT

A structure and method of making a thin-film solar cell is provided. A thin-film solar cell includes a substrate, absorber layer and a buffer layer. The absorber layer is deposited by a single-step bulk electrochemical process, or a multi-layer electrochemical process. The buffer layer is deposited by an electrochemical deposition process such as a multi-layer deposition or an atomic layer deposition. The absorber and buffer layers are non-toxic materials which can include sulfur incorporated during the deposition process or incorporated after deposition by an anneal step.

DOMESTIC PRIORITY

This application is a divisional application of U.S. patent applicationSer. No. 13/222,266, filed Aug. 31, 2011, which claims priority under 35U.S.C. §119(e) from U.S. Provisional Application No. 61/378,577 filed onAug. 31, 2010 the entire contents of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION

The present invention relates generally to electrochemical depositionmethods and, more particularly, to electrochemical deposition methodsfor depositing thin-film cells containing non-toxic elements.

Thin-film photovoltaic cells are made by depositing thin layers ofphotovoltaic material. The photovoltaic layers include p-type and n-typelayers. These layers can be made from a variety of photovoltaicmaterials and can be deposited using a variety of techniques.

The most commonly used materials for p-type layers include chalcopyritematerials, such as, copper indium gallium selenide (CIGS) and cadmiumtelluride (CdTe) which are toxic materials. The CIGS and CdTe layers arecommonly deposited using methods including co-evaporation, sputtering,electrodeposition and colloidal deposition. The most commonly usedmaterial for the n-type layer is cadmium sulfide (CdS) which is alsotoxic. The CdS layer is commonly deposited using either sputtering or achemical bath deposition.

There is a need to include a broader range of materials for use inthin-film photovoltaic cells. Specifically, there is a need forthin-film photovoltaic cells that use materials that are less toxic tothe environment and that contain elements that are low cost and are moreabundant in nature. In addition, there is a need for a process ofdepositing thin-film cells that offers low-cost processing and theability to introduce a variety of different alloy materials as p-typeand n-type layers.

BRIEF SUMMARY OF THE INVENTION

Accordingly, an aspect of the present invention provides a method ofmanufacturing thin-film cells. A substrate is provided. An absorberlayer is deposited onto the substrate. A buffer layer is deposited ontothe absorber layer using an electrochemical deposition process.

Another aspect of the present invention provides a thin-film cellstructure. The structure includes a substrate, an absorber layerdeposited onto the substrate using a single-step bulk electrochemicaldeposition process and a buffer layer deposited onto the absorber layerusing an electrochemical atomic layer deposition process.

Other features and advantages of the invention will become apparent fromthe detailed description and the accompanying drawing that follows.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 depicts a band diagram of p-CZTS at the liquid junction withaqueous solution under external bias of −0.35 V (vs. Ag/AgCl).

DETAILED DESCRIPTION OF THE INVENTION

Electrochemical deposition, including single-step bulk, multi-layeredand atomic layered methods, offers the advantages of being low costcompared to other deposition methods and allows for the introduction ofa variety of different quaternary, ternary and binary alloys asphotovoltaic materials. These methods of deposition can be combined inorder to produce efficient thin-film cells containing non-toxicmaterials with relatively low production costs.

The p-type layer, or absorber layer, is relatively thicker than then-type layer. The fast single-step bulk electrochemical depositionmethod is best suited for the absorber layer.

The n-type layer, or buffer layer, is relatively thinner than the p-typelayer. The slow process of electrochemical atomic layer deposition isbest suited for the buffer layer.

It is desirable to manufacture thin-film cells in photovoltaic deviceswithout using toxic materials. Accordingly, absorber and buffer layersthat do not contain cadmium (Cd) are proposed here. Further, fabricationof ternary absorber layers that do not contain cadmium (Cd), indium (In)or gallium (Ga) are also proposed.

A first aspect of the invention provides a method of manufacturingthin-film cells. The method includes providing a substrate, depositingan absorber layer onto the substrate, and depositing a buffer layer ontothe absorber layer using an electrochemical deposition process.

The step of depositing the absorber layer can be carried out usingeither (1) single-step bulk electrochemical deposition, (2) multi-stepelectrochemical deposition or (3) electrochemical atomic layerdeposition.

The step of depositing the buffer layer can be carried out using either(1) multi-step electrochemical deposition or (2) electrochemical atomiclayer deposition.

One advantage of electrochemical deposition is the ease to control thedriving force of the involved chemical reactions by simply adjusting theelectrode potential.

Referring to FIG. 1, a band diagram of p-CZTS at the liquid junctionwith aqueous solution under external bias of −0.35 V (vs. Ag/AgCl) isshown. By applying a more negative potential (see scale on the right),the Fermi level (E_(F)) of the electrode (dashed line) can be movedupwards, which can be considered roughly as the average energy of freeelectrons (i.e., with high tendency to take part in redox reactions).

For metal electrodes, when moving the Fermi level (E_(F)) above thepotential of a specified redox reaction in the solution phase (ex. thebar labeled by pH=3), electrons are injected into oxidative species ofthe redox couple (here is S₂O₃ ²⁻) and convert them into correspondingreductive species (in this example it is ZnS).

However, for a semiconductor electrode, things work differently becauseof the so-called band-bending which is illustrated for conduction band(CB) and valence band (VB) near the p-CZTS/solution interface changes.The band-bending forms because of the charge accumulation on thesolution side of the solid-liquid junction as well as the low freecharge carrier density of semiconductors.

In the dark, there are few electrons in the CB, and hence the onlypathway to inject electrons into the redox couple in solution and makethe deposition occur is thru the VB. As shown in FIG. 1, although theFermi level EF is higher than the redox potential of S₂O₃ ²⁻/ZnS in mostpH range between 3 and 11, the VB valence band edge is still lower thanthe redox potential by 0.3-0.7 V due to the band-bending. The depositioncannot occur due to the energy barrier of 300-700 mV.

In contrast, if light shines on the semiconductor electrode, electronson the VB can be excited into the CB, and then migrate downhill into CBedge at the interface, which is energetically higher than the redoxpotential of S₂O₃ ²⁻/ZnS. Therefore, the reaction can take placespontaneously under illumination at all pH ranges.

The absorber layer and the buffer layer can be made from non-toxicmaterials. The absorber layer material can include Cu₂ZnSeS₄, Cu₂SnSeS₄,Cu₂ZnSn(SeS)₄, Cu₂ZnSnTe₄, and Cu₂ZnSnS₄. Other materials that can workas either the absorber layer or the buffer layer can include In₂Se₃,In₂S₃, In₂Te₃, ZnSe, ZnTe, ZnS, SnSe, SnTe and SnS. Other materials thatcan also work are binary alloys including group II-VI materials, groupIII-V materials, group III-VI materials and group IV-VI materials.

The absorber materials containing sulfur can be deposited by includingsulfur in the solution bath during electrochemical deposition, or byannealing in the presence of sulfur (solid) and/or hydrogen sulfide(H₂S) in an Argon (Ar) containing atmosphere in order to implant sulfurinto the thin-film.

In one embodiment of the present invention, the absorber layer is athin-film of Cu₂ZnSn. This is carried out by contacting: (i) a solutionincluding: 1 mM to 100 mM of CuSO₄.5H₂O; 1 mM to 100 mM of SnSO₄; about1 mM to 100 mM of ZnSO₄; 20 mM to 5M of NaOH solution; and 0.1 M to 2 Mof sorbitol solution; and (ii) the substrate. The pH of the solution canbe adjusted by using more or less NaOH solution. The pH of the solutionshould range from about 9 to about 12. While the solution and thesubstrate are in contact, a voltage is applied to the substrate of about−0.4 V to about −1.5 V with reference to a Ag/AgCl reference electrode.The electric current evoked in the substrate should range from about 5mA/cm² to about 40 mA/cm².

If the solution used contains 1 mM to 100 mM of Na₂S, then the aboveprocess will produce a thin-film of Cu₂ZnSnS₄. Alternatively, thethin-film of Cu₂ZnSn can be annealed in the presence of sulfur (solid)and/or hydrogen sulfide (H₂S) in an Argon (Ar) containing atmosphere forabout 1 hour at a temperature of 550 degrees Celsius, thereby producinga thin-film of Cu₂ZnSnS₄.

In another embodiment of the present invention, the buffer layer is athin-film of In₂Se₃. The preferred method of depositing a buffer layerof In₂Se₃ is to use electrochemical atomic layer deposition. Thisprocess is carried out using a repeated process using an indiumsolution, a selenium solution and a blank rinse solution. The indiumsolution includes 1 mM to 500 mM InSO₄; 0.1 M to 1 M of NaClO₄; and 10mM to 1000 mM of C₂H₃NaO₂. The selenium solution includes 0.1 mM to 500mM of SeO₂; 0.1 to 1 M of NaClO₄; and about 10 mM to 100 mM of C₂H₃NaO₂.The blank rinse solution includes 0.1 M to 1 M of sodium perchlorate and10 mM to 100 mM of sodium acetate. All of the solutions have a pHadjusted to about 3 to about 10.

The process is carried out by filling a cell with the indium solution.The absorber layer is exposed to a light source. Then a voltage isapplied to the substrate of about −0.1 V to about −1 V with reference toa Ag/AgCl reference electrode for about 1 to 100 seconds. Whilemaintaining the voltage, the cell is rinsed for about 1 to 100 secondsusing the blank rinse solution. Then, the cell is filled with theselenium solution. The absorber layer is exposed to a light source. Thesame voltage is applied to the substrate of about −0.1 V to about −1 Vwith reference to the Ag/AgCl reference electrode for about 1 to 100seconds. Again, the cell is rinsed for about 1 to 100 seconds using therinse solution while the voltage is maintained.

The above process is repeated for 1 to 2000 cycles and for each cyclethe voltage applied to the electrode is increased by about −0.001 V upto a voltage of about −1 V. The end result is a thin-film of In₂Se₃.

In another preferred embodiment of the present invention, the bufferlayer is a thin-film of ZnS. The preferred method of depositing a bufferlayer of ZnS is to use electrochemical atomic layer deposition. Thisprocess is carried out using a repeated process using a zinc solution, asulfur solution and a blank rinse solution. The zinc solution includesabout 1 mM to 1000 mM ZnSO₄ buffered with NH₃. The sulfur solutionincludes about 0.1 mM to 1M Na₂S buffered with NH₃. The blank rinsesolution is an ammonia buffered solution. All of the solutions have a pHadjusted to about 9 to 12.

The process is carried out by filling a cell with the zinc solution. Theabsorber layer is exposed to a light source. Then a voltage is appliedto the substrate of about −0.4 V to about −1.5 V with reference to asaturated calomel reference electrode for about 1 to 100 seconds. Whilemaintaining the voltage, the cell is rinsed for about 1 to 100 secondsusing the blank rinse solution. Then, the cell is filled with the sulfursolution. The absorber layer is exposed to a light source. The samevoltage is applied to the substrate of about −0.4 V to about −1.5 V withreference to the Ag/AgCl reference electrode for about 1 to 100 seconds.Again, the cell is rinsed for about 1 to 100 seconds using the rinsesolution and the voltage is maintained.

The above process is repeated for 1 to 2000 cycles and for each cyclethe voltage applied to the electrode is increased by about −0.001 V upto a voltage of about −1 V or by about +0.001 V down to a voltage ofabout −0.1 V. The end result is a thin-film of ZnS.

In another preferred embodiment of the present invention, the absorberlayer is a thin-film of SnS. The thin-film of SnS is deposited byfilling a cell with a solution including: SnCl₂; Na₂S₂O₃; and about 0.2mM of tartaric acid. The pH of the solution is adjusted to about 2.5.The temperature of the solution is adjusted to about 70 degrees Celsius.The absorber layer is exposed to a light source. A voltage is applied toa substrate in the cell. The voltage applied should be about −0.8 V withreference to a Ag/AgCl reference electrode.

The thin-film structure includes a substrate, an absorber layerdeposited onto the substrate using a single-step bulk electrochemicaldeposition process and a buffer layer deposited onto the absorber layerusing an electrochemical atomic layer deposition process. The thicknessof the thin-film cell ranges from about 50 nm to about 3 μm. Theabsorber layer and the buffer layer are made from non-toxic materials.The absorber layer material can include Cu₂ZnSeS₄, Cu₂SnSeS₄,Cu₂ZnSn(SeS)₄, Cu₂ZnSnTe₄ or Cu₂ZnSnS₄. The absorber layer and thebuffer layer material can include In₂Se₃, In₂S₃, In₂Te₃, ZnSe, ZnTe,ZnS, SnSe, SnTe or SnS.

While the present invention has been described with reference to whatare presently considered to be the preferred embodiments, it is to beunderstood that the invention is not limited to the disclosedembodiments. On the contrary, the invention is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims. The scope of the following claims is to beaccorded the broadest interpretation so as to encompass all suchmodifications and equivalent structures and functions.

What is claimed is:
 1. A method of manufacturing a thin-film cell, the method comprising: providing a substrate; depositing an absorber layer onto the substrate; and depositing a buffer layer onto the absorber layer, wherein the absorber layer and the buffer layer are made from non-toxic materials; wherein the depositing the absorber layer and the buffer layer is performed using an electrochemical atomic layer deposition process carried out under illumination so as excite electrons from a valence band to a conduction band, wherein the electrons at an edge of the conduction band at a solid/liquid interface are energetically higher than that of a redox potential of S₂O₃ ²⁻/ZnS; and wherein the step of depositing the absorber layer onto the substrate is carried out by depositing a thin-film of Cu₂ZnSnS₄ using an electrochemical deposition process, wherein sulfur is contained in a solution used in the process.
 2. The method of claim 1, wherein the step of depositing the absorber layer onto the substrate is carried out using a single-step bulk electrochemical deposition process.
 3. The method of claim 1, wherein the step of depositing the absorber layer onto the substrate is carried out using multiple-layer electrochemical deposition.
 4. The method of claim 1, wherein the step of depositing the absorber layer onto the substrate is carried out using a single-step bulk electrochemical deposition process and wherein the electrochemical deposition process used to deposit the buffer layer is a multiple layer electrochemical deposition process. 